In the treatment of patients suffering from various respiratory problems, either of acute or chronic nature, involving the lungs, bronchia, etc., it is often necessary to test the breathing capacity of the patient. Prior art devices or apparatus for this purpose have tended to either be quite expensive or of a low degree of accuracy. An improved inhalation valve for such purposes, and which has attained a substantial degree of commercial acceptance, is disclosed in our prior application Ser. No. 394,403, filed July 1, 1982 for "Inhalation Valve". In our prior inhalation valve, a restricted inlet passage was provided in which a reduced pressure was developed upon inhalation. This inlet passage was displaced to one side of the main passage to which it was joined by a tortuous flow path. A single diaphragm or flapper valve closed off the main passage upon inhalation and allowed it to open upon exhalation.